Seems the Colour Picker Tool is missing from the default set of tools on Affinity Photo Beta for Windows. You can add it going to menu View ▸ Customise Tools.... On the Mac version it appears between the Move Tool and the Crop Tool. I'm logging this.
As far as I can see, the ability to preview exported images is, as yet, only a roadmap item for AD. It's one of the missing features that will mean I won't purchase the software.
The AD 1.5 blurb at https://affinity.serif.com/en-gb/ui-design-software/announces that AD is "the perfect tool for UI and UX design". But without any way to preview and/or assess the quality of the output images, isn't it difficult to use AD for these purposes? Seems to me that you have to adopt the trial-and-error approach.
But maybe I've misunderstood how we're expected to work. What do other users do in the absence of an export preview feature?
Brian Jackson reacted to MEB in "color picker?"